1. Field of the Invention
The present invention relates to an image forming apparatus, such as an electrophotographic copier, printer, facsimile, or multifunction apparatus combining two or more of these functions.
2. Description of the Background
Image forming apparatuses such as full-color copiers or printers that employ an intermediate transfer member are widely used. In such an image forming apparatus, multiple toner images are superimposed on one another on the intermediate transfer member in a primary transfer process, and the resulting composite toner image is then transferred onto a recording medium in a secondary transfer process.
The intermediate transfer member generally comprises a low-resistivity material, a high-resistivity material, or a combination thereof. In a case where the intermediate transfer member comprises a high-resistivity material, an electric field applied thereto can be suppressed from spreading because charges are not easily movable therein. In this case, toner particles can be normally transferred onto the intermediate transfer member without causing toner scattering or producing low granularity images. However, when such an intermediate transfer member comprising a high-resistivity material is subjected to continuous image formation, charges are likely to remain and accumulate within the intermediate transfer member, causing charge-up on the surface. Also, such an intermediate transfer member comprising a high-resistivity material generally requires a high bias voltage, which causes variation in surface potential of the intermediate transfer member among portions bearing leading and trailing edges of a recording medium, a large amount of toner particles, or a small amount of toner particles. This variation in surface potential persists through time (so-called potential history or potential memory) and produces residual images (ghosts) in the primary and secondary transfer processes.
Japanese Patent Application Publication No. (hereinafter JP-A) 2008-3522 discloses an image forming apparatus employing a transfer device which removes charges from an intermediate transfer member by contacting a conductive brush, to which a bias having the opposite polarity to the surface potential of the intermediate transfer member is applied, with the intermediate transfer member after the secondary transfer process. However, it is difficult for the conductive brush to completely remove the charges from the surface and equalize the surface potential at zero. In particular, in a case where a high transfer bias is applied for transferring a toner image onto a thick sheet of paper, the variation in surface potential cannot be completely removed.
JP-2006-267951-A and JP-H08-160771-A each disclose an image forming apparatus employing a transfer device which removes charges from an intermediate transfer member by emitting light onto the intermediate transfer member after the secondary transfer process. It is difficult to completely remove the charges from the surface by emission of light, however, and some localized charges are likely to remain on the surface of the intermediate transfer member. As a result, residual images are produced in the primary and secondary transfer processes. JP-2006-267951-A is also disadvantageous in that the plurality of intermediate transfer members employed, one for each color, makes the image forming apparatus complicated and requires a large space.
JP-H11-167294-A discloses an image forming apparatus employing a transfer device including an intermediate transfer belt having a high-resistivity layer. The high-resistivity layer controls the current value injected into the intermediate transfer member to prevent charge-up thereof. However, such a high-resistivity layer cannot completely prevent the occurrence of charge-up in the secondary transfer area, resulting in production of abnormal images.
JP-2004-279571-A discloses an image forming apparatus employing a transfer device including an intermediate transfer belt, which satisfies the inequation: |surface potential just before secondary transfer|≧|surface potential just after secondary transfer−surface potential just before secondary transfer|. Such a transfer device prevents production of residual images that results from the above-described residual surface potential history of the intermediate transfer member. However, the above inequation is satisfied only when the secondary transfer current is relatively small. Therefore, rough-surface paper, which requires a relatively large transfer current, cannot be used in the above transfer device.
Japanese Patent No. 4175714 (corresponding to JP-2000-231278-A) discloses an image forming apparatus employing a transfer device including an intermediate transfer member having a thin insulative surface layer, the thickness of which is set to 1 μm or less, to reduce residual potential. However, such a thin insulative layer has poor abrasion resistance and insulation, thereby producing abnormal images by charge leakage.